The present invention relates to a method for instantly killing fish and at the same time cooling them so as to keep them very fresh for a long time.
A method for keeping fish very fresh by instantly killing them and for putting tasty fish on the market has been well known.
The prevent invention relates to a method for keeping the fish very fresh for a long time using a new method for instantly killing the fish.
More particularly, the present invention relates to a method for processing very fresh fish and frozen fish by instantly killing and effectively cooling a large quantity of abundantly available popular sea fish such as sardines, horse mackerel, mackerel and mackerel pike or popular freshwater fish from varieties such as salmon and sweetfish.
For a method for treating fish to keep them very fresh and tasty for a long time, especially important are:
(1) to make the fish die from shock in a short time without agony, and
(2) to cool the fish rapidly in a short time preferably without freezing them because in a bad frozen state, the fish flesh is degraded.
In order to satisfy these conditions, high quality fish such as tunas, porgys and young yellowtails are instantly killed by stabbing the hindbrain in the head of the fish immediately after they are caught, and immediately cooled, for example, in ice water to preserve freshness.
In this process, unless the stab hits accurately the hindbrain of the fish, the fish is not instantly killed and it loses its commercial value. Therefore, in almost all cases, the fish are killed by human hand.
The fish treated by the method of the present invention are relatively small-sized popular fish which are caught a lot at a time. These fish can be hardly treated effectively in a short time by such a manual operation.
A lot of popular fish caught with a round haul net, a draw net or a fixed shore net are generally treated by the ice water method, transported immersed in the ice water to the fishing port and then landed with a landing net or by a water pump. The degree of freshness of the fish landed in such a manner varies widely from fish tank to fish tank. Besides, since the fish died with agony, copious bleeding from the gills or the body surface is observed, and the taste and thus the commercial value of the fish are lowered.
In order to research this, the inventor has observed in detail the lifting of the fish from net into the fish tank on. As a result, it has been observed that when the fish are thrown into the cooling sea water obtained by mixing sea water with crushed ice originally in the fish tank, they do not immediately die from shock but most of fish are leaping on the crushed ice and still alive for several minutes.
Live fish dipped into the cooling sea water die rather soon, but they are alive for several minutes in the cool water at a temperature of 0 degree C. + or - several degrees. In other words, the fish are in agony in the cooling water, and are not instantly killed from shock.
There was copious bleeding from the gills and body surface of the fish dying in agony and the ice water was colored dark-red when landed and their excrements and the like were observed in it.
In the body of the fish dying in agony and struggle, the muscles are convulsed or temporarily contract. At this time, a large amount of energy is wasted, and in the muscles, glycogen is decomposed to accumulate lactic acid. Therefore, the pH of the fish body is acidified and the heat generation due to the self-digestion of the fish causes the temperature to rise, whereby the freshness degradation in the early stage is remarkably promoted. Besides, the taste of the fish flesh becomes worse.
Further, though fish are cooled in ice water in the fish tank, the temperature of the ice water is uneven in parts, and the degree of freshness of the fish varies widely. It has been observed that all the fish cannot be uniformly cooled in a short time, and the temprature of a part of the fish rises to 4 degrees C. or 5 degrees C., and that in the case of the actively self-digesting fish such as walleye pollacks, the temperature of the inside part of the fish body rises as high as to 15 degrees C. even though there is ice on the water surface in the fish tank.
Unfortunately as the result, in the conventional art, it is very difficult to keep the nettable en masse popular fish in a flavorful and very fresh state for a long time.
In comparison with the large-sized high quality fish. the small-sized nettable en masse popular fish are especially apt to self-digest and to be degraded in freshness. Therefore, a more advanced art is required for keeping the popular fish fresh for a long time, in comparison with the case of the high quality fish.
The inventor has discovered that the fish, rapidly cooled by the generally used method of dipping the fish in the cool sea water mixed with freshwater ice (the ice water method), are put in agony for a much longer time than the case where they are instantly killed by stabbing their hindbrain.
While the fish stabbed in the hindbrain die at least within a few seconds, the fish cooled by the ice water method are in agony for a few minutes. The inventor has paid attention to the difference between the times until the fish die.
After that, the inventor has made an experiment. When living gray mullets and filefishs were thrown into ice water at a temperature of 0 degree C., it was obviously observed that the gray mullets and the filefishs were breathing and alive for 40 minutes and an hour respecteivly. Consequently, it has been confirmed that, according to the conventional way of instantly killing fish by the ice water method, the fish die not from rapid cooling but from suffocation due to oxygen starvation or the like caused by the throwing of a lot of fish into the fish tank.